Intracellular receptors are a class of structurally related proteins involved in the regulation of gene proteins. Steroid receptors are a subset of these receptors, including the progesterone receptors (PR), androgen receptors (AR), estrogen receptors (ER), glucocorticoid receptors (GR) and mineralocorticoid receptors (MR). Regulation of a gene by such factors requires the intracellular receptor and corresponding ligands, which has the ability to selectively bind to the receptor in a way that affects gene transcription.
Progesterone receptor modulators (progestagens) are known to play an important role in mammalian development and homeostasis. Progesterone is known to be required for mammary gland development, ovulation and the maintenance of pregnancy. Currently, steroidal progestin agonists and antagonists are clinically approved for contraception, hormone replacement therapy (HRT) and therapeutic abortion. Moreover, there is good preclinical and clinical evidence for the value of progestin antagonists in treating endometriosis, uterine leiomyomata (fibroids), dysfunctional uterine bleeding and breast cancer.
The current steroidal progestagens have been proven to be quite safe and are well tolerated. Sometimes, however, side effects (e.g. breast tenderness, headaches, depression and weight gain) have been reported that are attributed to these steroidal progestagens, either alone or in combination with estrogenic compounds.
Steroidal ligands for one receptor often show cross-reactivity with other steroidal receptors. As an example, many progestagens also bind to glucocorticoid receptor. Non-steroidal progestagens have no molecular similarity with steroids and therefore one might also expect differences in physicochemical properties, pharmacokinetic (PK) parameters, tissue distribution (e.g. CNS versus peripheral) and, more importantly, non-steroidal progestagens may show no/less cross-reactivity to other steroid receptors. Therefore, non-steroidal progestagens will likely emerge as major players in reproductive pharmacology in the foreseeable future.
It was known that progesterone receptor existed as two isoforms, full-length progesterone receptor isoform (PR-B) and its shorter counterpart (PR-A). Recently, extensive studies have been implemented on the progesterone receptor knockout mouse (PRKO, lacking both the A- and B-forms of the receptors), the mouse knockouts specifically for the PR-A isoform (PRAKO) and the PR-B isoform (PRBKO). Different phenotypes were discovered for PRKO, PRAKO and PRBKO in physiology studies in terms of fertility, ovulation uterine receptivity, uterine proliferation, proliferation of mammary gland, sexual receptivity in female mice, sexual activity in male mice and infanticide tendencies in male mice. These findings provided insights for synthetic chemists to construct not only selective progesterone receptor modulator (SPRM), but also PR-A or PR-B selective progesterone receptor modulator.
Progesterone plays a major role in reproductive health and functioning. Its effects on, for example, the uterus, breast, cervix and hypothalamic-pituitary unit are well established. The actions of progesterone as well as progesterone antagonists are mediated by the progesterone receptor (PR). In the target cell, progesterone produces a dramatic change in confirmation of the PR that is associated with transforming the PR from a non-DNA binding form to one that will bind to DNA. This transformation is accompanied by a loss of associated heat shock proteins and dimerization. The activated PR dimmer then binds to specific DNA sequences within the promotor region of progesterone responsive genes. The agonist-bound PR is believed to activate transcription by associating with coactivators, which act as bridging factors between the receptor and the general transcription machinery. This is followed by increases in the rate of transcription producing agonist effects at the cellular and tissue levels. These progesterone receptor ligands exhibit a spectrum of activity ranging from pure antagonists to mixed agonists/antagonists.
In 1982, the discovery of compounds that bind to the progesterone receptor, antagonize the effects of progesterone receptor and antagonize the effects of progesterone was announced. Although compounds such as estrogens and certain enzyme inhibitors can prevent the physiological effects of endogenous progesterone, the term “antiprogestin” is confined to those compounds that bind to the progestin receptor. A report from the Institute of Medicine (Donaldson, Molly S.; Dorflinger, L.; Brown, Sarah S.; Benet, Leslie Z., Editors, Clinical Applications of Mifepristone (RU 486) and Other antiprogestins, Committee on antiprogestins: Assessing the science, Institute of medicine, National Academy Press, 1993) summarized a number of medical conditions related to the effect of antiprogestins. In view of the pivotal role that progesterone plays in reproduction, it is not surprising that antiprogestins could play a part in fertility control, including contraception, menses induction and medical termination of pregnancy, but there are many other potential uses that have been supported by small clinical or preclinical studies, such as labor and delivery; treatment of uterine leiomyomas (fibroids), treatment of endometriosis; HRT; breast cancers; male contraception, etc.
The effects and uses of progesterone agonists have been well established. In addition, it has been recently shown that certain compounds structurally related to the known antiprogestins have agonist activity in certain biological systems (e.g., the classical progestin effects I the estrogen-primed immature rabbit uterus; cf. C. E. Cook et al., Life Sciences, 52, 155-162 (1993)). Such compounds are partial agonists in human cell-derived receptor systems, where they bind to a site distinct from both the progestin and antiprogestin sites (Wagner et al., Proc. Natl. Acad. Sci., 93, 8739-8744 (1996)). Thus the general class of antiprogestins can have subclasses, which may vary in their clinical profiles.
Compounds which mimic some of the effects of progesterone (agonists), antagonize these effects (antagonists, antiprogestins) or exhibit mixed effects (partial agonists or mixed agonist/antagonist), known as progesterone receptor modulators (PRMs) can be useful in treating a variety of disease states and conditions. PR agonists have been used in female contraceptives and in postmenopausal hormone therapy. Recent studies in women and non-human primates show that PR antagonists may also have potential as contraceptive agents and for the treatment of various gynecological and obstetric diseases, including fibroids, endometriosis and, possibly, hormone-dependent cancers. Clinically available PR agonists and antagonists are steroidal compounds and often cause various side effects due to their functional interaction with other steroid receptors. Recently, numerous receptor-selective non-steroidal PR agonists and antagonists have emerged. Non-steroidal PR antagonists, being structurally distinct from the steroid class, may have greater potential for selectivity against other steroid receptors.